System and method for handover to minimize service delay due to ping pong effect in BWA communication system

ABSTRACT

Disclosed is a method for recognizing by a Mobile Subscriber Station (MSS) whether a serving Base Station (BS) retains connection information in a mobile communication system including the serving BS for providing a service to the MSS and a target BS adjacent to the serving BS. The method includes the steps of receiving a handover request message from the serving BS, the handover request message including a resource remain type field representing whether the connection information having been set with the serving BS is retained, and a resource retain time field representing a connection information-retaining time by the serving BS, and recognizing that the serving BS retains the connection information during a time determined by a value of the resource retain time field.

PRIORITY

This application claims priority to two applications entitled “Systemand Method for Handover to minimize Service Delay due to Ping PongEffect in BWA Communication System” filed in the Korean IntellectualProperty Office on Mar. 5, 2004 and assigned Serial No. 2004-15217, andfiled on May 7, 2004 and assigned Serial No. 2004-32215, the contents ofeach of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Broadband Wireless Access (BWA)communication system, and more particularly to a system and a method forperforming a handover, that can minimize a service delay due to apingpong effect.

2. Description of the Related Art

In a 4th generation (4G) communication system, which is the nextgeneration communication system, research has been actively pursued toprovide users with services having various qualities of service (QoS) ata transmission speed of about 10 Mbps. In particular, in the current 4Gcommunication system, research has been actively pursued to support ahigh speed service capable of ensuring mobility and QoS in a BWAcommunication system such as a wireless local area network (LAN) systemand a wireless metropolitan area network (MAN) system. A representativecommunication system of the 4G communication system is based on an IEEE(Institute of Electrical and Electronics Engineers) 802.16acommunication system standard and an IEEE 802.16e communication systemstandard.

The IEEE 802.16a communication system and the IEEE 802.16e communicationsystem are communication systems employing an Orthogonal FrequencyDivision Multiplexing (OFDM) scheme/an Orthogonal Frequency DivisionAccess (OFDMA) scheme in order to support a broadband transmissionnetwork for a physical channel of the wireless MAN system. The IEEE802.16a communication system only takes into consideration a single cellstructure and stationary Subscriber Stations (SSs), which means thesystem does not in any way reflect the mobility of a SS. In contrast,the IEEE 802.16e communication system reflects the mobility of an SS inthe IEEE 802.16a communication system. Here, an SS having the mobilityis referred to as a Mobile Subscriber Station (MSS).

The structure of the IEEE 802.16e communication system will be describedwith reference to FIG. 1.

FIG. 1 is a block diagram showing the general structure of the IEEE802.16e communication system.

Referring to FIG. 1, the IEEE 802.16e communication system has amulti-cell structure, that is, a cell 100 and a cell 150. Also, the IEEE802.16e communication system includes a Base Station (BS) 110controlling the cell 100, a BS 140 controlling the cell 150, and aplurality of MSSs 111, 113, 130, 151, and 153. Thetransmission/reception of signals between the BSs 110 and 140 and theMSSs 111, 113, 130, 151, and 153 is accomplished through an OFDM/OFDMAmethod. The MSS 130 from among the MSSs 111, 113, 130, 151, and 153 islocated in a boundary area (i.e., handover area) between the cell 100and the cell 150. When the MSS 130 moves into the cell 150 controlled bythe BS 140 during the transmission/reception of signals with the BS 110,a serving BS of the MSS 130 changes from the BS 110 to the BS 140.

FIG. 2 is a flow diagram illustrating a general handover process by therequest of the MSS in the IEEE 802.16e communication system.

Referring to FIG. 2, a serving BS 210 transmits a Mobile NeighborAdvertisement (MOB_NBR_ADV) message to an MSS 200 (step 211). TheMOB_NBR_ADV message has a structure as shown in Table 1. TABLE 1 SyntaxSize Notes MOB_NBR_ADV_message_Format( ) { Management Message Type=48 8bits Operator ID 24 bits Unique ID assigned to the operatorConfiguration Change Count 8 bits N_NEIGHBORS 8 bits For (j=0;j<N_NEIGHBORS;J++){ Neighbor BS-ID 48 bits Physical Frequency 32 bits TLV(Type Length Variable) Variable TLV specific Encoded NeighborInformation } }

As shown in Table 1, the MOB_NBR_ADV message includes a plurality ofInformation Elements (IEs), that is, the ‘Management Message Type’representing the type of a transmitted message, the ‘Operator ID’representing a network identifier (ID), the ‘Configuration Change Count’representing the number of times by which a configuration changes, the‘N_NEIGHBORS’ representing the number of neighbor BSs, the ‘NeighborBS-ID’ representing IDs of the neighbor BSs, the ‘Physical Frequency’representing the physical frequency of the neighbor BS, and the ‘TLVEncoded Neighbor Information’ representing extra information relating tothe neighbor BS in addition to the information.

The MSS 200 may acquire information for the neighbor BSs by receivingthe MOB_NBR_ADV message. Further, when the MSS 200 intends to scan theCINRs (Carrier to Interference and Noise Ratios) of the pilot channelsignals transmitted from the neighbor BSs and the serving BS 210, theMSS 200 transmits a Mobile Scanning Interval Allocation Request(MOB_SC-REQ) message to the serving BS 210 (step 213). The MOB_SCN-REQmessage has a structure as shown in Table 2. TABLE 2 Syntax Size NotesMOB_SCN_REQ_message_Format( ) { Management Message Type=? 8 bits ScanDuration 12 bits Units are frames Start Frame 4 bits }

As shown in Table 2, the MOB_SCN_REQ message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, the ‘Scan Duration’ representing a scan durationfor the CINRS of the pilot channel signals transmitted from the neighborBSs, and the ‘Start Frame’ representing a start frame for a scanningoperation. The ‘Scan Duration’ is constructed by the frame. In Table 2,the ‘Management Message Type’ of the MOB_SCN_REQ message has not yetbeen defined (i.e., Management Message Type=undefined). Herein, since atime point at which the MSS 200 requests the scan has no directconnection with the scanning operation for the CINRs of the pilotchannel signals, the detailed description will be omitted.

Meanwhile, the serving BS 210 having received the MOB_SCN_REQ messagetransmits a Mobile Scanning Interval Allocation Response (MOB_SCN_RSP)message, which includes information to be scanned by the MSS 200 andincludes a scan duration having values except for 0, to the MSS 200(step 215). The MOB_SCN_RSP message has a structure as shown in Table 3.TABLE 3 Syntax Size Notes MOB_SCN_RSP_message_Format( ) { ManagementMessage Type=? 8 bits For(i=0;i<num_CIDs;i++){ num_CIDs can bedetermined from the length of the message {(found in the generic MAC(Medium Access Control) header) CID 16 bits basic CID of the MSSDuration 12 bits in frames Estimated time for 8 bits hand-over StartFrame 4 bits } }

As shown in Table 3, the MOB_SCN_RSP message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, the CID (Connection ID) of the MSS havingtransmitted the MOB_SCN_REQ message, the ‘Duration’, and the startingpoint of a scanning operation. In Table 3, the ‘Management Message Type’of the MOB_SCN_RSP message to be transmitted has not yet been defined(i.e., Management Message Type=undefined), and the ‘Duration’ representsa duration for which the MSS 200 performs the pilot CINR scanning. Avalue of 0 for the Duration indicates a rejection of the scan request ofthe MSS 200.

The MSS 200 having received the MOB_SCN_RSP message containing thescanning information scans the CINRs of the pilot channel signals forneighbor BSs and the serving BS 210, which have been recognized throughthe reception of the MOB_NBR_ADV message, according to the parameters(i.e., Duration) contained in the MOB_SCN_RSP message (step 217).

After having completed the scanning for the CINRs of the pilot channelsignals received from the neighbor BSs and the serving BS 210, when theMSS 200 determines to change the serving BS to which the MSS 200currently belongs (step 219), that is, the MSS 200 determines to changethe current serving BS to a new BS different from the BS 210, the MSS200 transmits a Mobile Subscriber Station HandOver Request(MOB_MSSHO_REQ) message to the serving BS 210 (step 221). Herein, thenew BS, which is not the serving BS currently including the MSS 200 buta BS capable of being a new serving BS through the handover of the MSS200, is referred to as a target BS. The MOB_MSSHO_REQ message has astructure as shown in Table 4. TABLE 4 Syntax Size NotesMOB_MSSHO_REQ_message_Format( ) { Management Message Type=52 8 bits For(j=0;j< N_Recommended;J++){ N_Recom- mended can be derived from theknown length of the message Neighbor BS-ID 48 bits BS S/(N+1) 8 bitsService level prediction 8 bits Estimated HO Time 8 bits } }

As shown in Table 4, the MOB_MSSHO_REQ message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, and a result obtained by scanning the CINRs of thepilot channel signals of the serving BS and the neighbor BSs by the MSS200. In Table 4, the ‘N_Recommended’ represents the number of neighborBSs having transmitted the pilot channel signals, which have CINRslarger than preset CINRs, on the basis of the scanning result for theCINRs of the pilot channel signals of each neighbor BS. The‘N_Recommended’ represents the number of neighbor BSs recommended as BSsto which the MSS 200 could be handed overto. The MOB_MSSHO_REQ messageincludes the IDs for each neighbor BS, which are represented by the‘N_Recommended’, the CINRs of the pilot channel signals for eachneighbor BS, the service level predicted to be provided to the MSS 200by the neighbor BSs, and the Estimated HO Time predicted as the startingtime of handover after the neighbor BS is selected as a target BS.

When the serving BS 210 receives the MOB_MSSHO_REQ message transmittedfrom the MSS 200, the serving BS 210 detects a list of possible targetBSs to which the MSS 600 could be handed overto by means of the‘N_Recommended’ information of the received MOB_MSSHO_REQ message (step223). For convenience of description, the list of target BSs to whichthe MSS 600 could be handed overto will be called a ‘handover-executabletarget BS list’. In FIG. 2, it is assumed that a first target BS 220 anda second target BS 230 exist in the handover-executable target BS list.Also, the handover-executable target BS list may include a plurality oftarget BSs. The serving BS 210 transmits a HO_PRE_NOTIFICATION messageto the target BSs (i.e., the first target BS 220 and the second targetBS 230) contained in the handover-executable target BS list (steps 225and 227). The HO_PRE_NOTIFICATION message has a structure as shown inTable 5. TABLE 5 Field Size Notes Global Header 152-bit  For (j=0;j< NumRecords;J++){ MSS unique identifier 48-bit 48-bit unique identifier usedby MSS (as provided by the MSS or by the I-am- host-of message)Estimated Time to HO 16-bit In milliseconds, relative to the time stamp,value 0 of this parameter indicates that no actual HO is pendingRequired BW  8-bit Bandwidth which is required by MSS (to guaranteeminimum packet data transmission) Required QoS  8-bit Name of ServiceClass representing Authorized QoSparamSet } Security field TBD A meansto authenticate this message CRC field 32-bit IEEE CRC-32

As shown in Table 5, the HO_PRE_NOTIFICATION message includes aplurality of IEs, that is, the Global Header commonly contained inmessages exchanged between BSs in a backbone network, the MSS ID of theMSS 200 which is to be handed over to the first target BS 220 or thesecond target BS 230, the Estimated HO Time representing a timepredicted as a start time of the handover by the MSS 200, the RequiredBW representing information on a bandwidth requested from the MSS 200 toa target BS to be a new serving BS, the Required QoS representinginformation on the level of a service to be provided to the MSS 200,etc. The bandwidth and the service level requested by the MSS 200 areidentical to the predicted service level information recorded in theMOB_MSSHO_REQ message described in table 4.

The Global Header, which is commonly contained in the messages (equal tothe HO-PRE-NOTIFICATION message) exchanged between the BSs in thebackbone network, has a general structure as shown in Table 6. TABLE 6Field Size Notes Message Type=?  8-bit Sender BS-ID 48-bit Base stationunique identifier (Same number as broadcast on the DL_MAP message)Target BS-ID 48-bit Base station unique identifier (Same number asbroadcast on the DL_MAP message) Time Stamp 32-bit Number ofmultiseconds since midnight GMT (set to 0xffffffff to ignore) NumRecords 16-bit Number of MSS identity records

As shown in Table 6, the Global Header includes a plurality of IEs, thatis, the ‘Message Type’ representing the type of a transmitted message,the Sender BS-ID representing a transmission BS transmitting themessage, the Target BS-ID representing a reception BS receiving themessage, and the Num Records representing the number of MSSs which aresubjects of a record included in the message.

When the first target BS 220 and the second target BS 230 receive theHO_PRE_NOTIFICATION message from the serving BS 210, they transmit aHO_PRE_NOTIFICATION_RESPONSE message, which is a response message forthe HO_PRE_NOTIFICATION message, to the serving BS 210 (steps 229 and231). The HO_PRE_NOTIFICATION-RESPONSE message has a structure as shownin Table 7. TABLE 7 Field Size Notes Global Header 152-bit For (j=0;j<Num_Records;J++){ MSS unique identifier  48-bit 48-bit unique identifierused by MSS (as provided by the MSS or by the I- am-host-of message) BWEstimated  8-bit Bandwidth which is pro- vided by BS (to guaran- teeminimum packet data transmission) TBD how to set this field QoSEstimated  8-bit Quality of Service level Unsolicited Grant Service(UGS) Real-time Polling Service (rtPS) Non-Real-time Polling ServicenrtPS) Best Effort ACK/NACK  8 bits Acknowledgement or Negativeacknowledgement 1 is Acknowledgement which means that the neighbor BSaccepts the HO-pre-notification message from the serving BS 0 isNegative Acknow- ledgement which means that the neighbor BS may notaccept the HO-pre- notification message from the serving BS } Securityfield TBD A means to authenticate this message CRC field  32-bit IEEECRC-32

As shown in Table 7, the HO_PRE_NOTIFICATION_RESPONSE message includes aplurality of IEs, that is, the Global Header commonly included in themessages exchanged between the BSs in the backbone network as describedin table 6, the MSS ID of an MSS which is to be handed over to targetBSs, the ACK/NACK regarding whether the target BSs can perform handoveraccording to the handover request of the MSS, and the bandwidth andservice level information capable of being provided by each target BSwhen the MSS is handed over to each target BS.

Meanwhile, the serving BS 210 having received theHO_PRE_NOTIFICATION_RESPONSE messages from the first target BS 220 andthe second target BS 230 analyzes the receivedHO_PRE_NOTIFICATION_RESPONSE message, and selects a target BS, which canoptimally provide the bandwidth and the service level requested by theMSS 200 when the MSS 200 has been handed over, as a final target BS towhich the MSS 200 is to be handed over. For instance, when it is assumedthat the service level capable of being provided by the first target BS220 is less than that requested by the MSS 200, and the service levelcapable of being provided by the second target BS 230 is identical tothat requested by the MSS 200, the serving BS 210 selects the secondtarget BS 230 as the final target BS to which the MSS 200 is to behanded over. Accordingly, the serving BS 210 transmits a HO_CONFIRMmessage to the second target BS 230 as a response message for theHO_PRE_NOTIFICATION_RESPONSE message (step 233). The HO_CONFIRM messagehas a structure as shown in Table 8. TABLE 8 Field Size Notes GlobalHeader 152-bit For (j=0;j< Num_Records;J++){ MSS unique identifier48-bit 48-bit universal MAC address of the MSS (as provided to the BS onthe RNG-REQ message) BW Estimated 8-bit Bandwidth which is provided byBS (to guarantee minimum packet data trans- mission) TBD how to set thisfield QoS Estimated 8-bit Quality of Service level Unsolicited GrantService (UGS) Real-time Polling Service (rtPS) Non-Real-time PollingService (nrtPS) Best Effort Service (BE) } Security field TBD A means toauthenti- cate this message CRC field 32-bit IEEE CRC-32

As shown in Table 8, the HO_CONFIRM message includes a plurality of IEs,that is, the Global Header commonly included in the messages exchangedbetween the BSs in the backbone network as described in table 6, an MSSID of an MSS which is to be handed over to the selected target BS, andthe bandwidth and service level information capable of being providedfrom the target BS when the MSS is handed over to the selected targetBS.

Also, the serving BS 210 transmits a BS HandOver Response (MOB_BSHO_RSP)message to the MSS 200 as a response message for the MOB_MSSHO_REQmessage (step 235). Herein, the MOB_BSHO_RSP message includesinformation on the target BS to which the MSS 200 is to be handed over.The MOB_BSHO_RSP message has a structure as shown in Table 9. TABLE 9Syntax Size Notes MOB_BSHO_RSP_message_Format( ) { Management MessageType=53  8 bits Estimated HO time  8 bits For (j=0;j< Neighbor baseN_Recommended;J++){ stations shall be presented in an order such thatthe first presented is the one most recommended and the last pre- sentedis the least recom- mended. N_Recommended can be derived from the knownlength of the message. Neighbor BS-ID 48 bits Service level prediction 8 bits } }

As shown in Table 9, the MOB_BSHO_RSP message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, the Estimated HO time predicted as a start time ofa handover procedure, and the result for the target BSs selected by theserving BS. Further, the ‘N_Recommended’ of the MOB_BSHO_RSP messagerepresents the number of target BSs, which satisfy the bandwidth and theservice level requested by the MSS, among target BSs on thehandover-executable target BS list. The MOB_BSHO_RSP message includesthe IDs of each target BS represented by the ‘N_Recommended’ and thelevel of the service predicted to be provided from each target BS to theMSS. In FIG. 2, the MOB_HO_RSP message finally includes only informationon the second target BS 230 from among the target BSs on thehandover-executable target BS list. However, if there exist multipletarget BSs capable of providing the bandwidth and service levelrequested by the MSS 200 from among the target BSs on thehandover-executable target BS list, the MOB_BSHO_RSP message includesinformation on the multiple target BSs.

Further, the MSS 200 having received the MOB_BSHO_RSP message analyzesthe ‘N_Recommended’ information contained in the MOB_BSHO_RSP messageand selects the target BS to which the MSS 200 is to be handed over.Then, the MSS 200 having selected the target BS to which the MSS 200 isto be handed over transmits a Mobile Handover Indication (MOB_HO_IND)message to the serving BS 210 as a response message for the MOB_BSHO_RSPmessage (step 237). The MOB_HO_IND message has a structure as shown inTable 10. TABLE 10 Syntax Size Notes MOB_HO_IND_message_Format( ) {Management Message Type=54 8 bits reserved 6 bits Reserved; shall be setto zero HO_IND_type 2 bits 00: Serviing BS release 01: HO cancel 10: HOreject 11: reserved Target_BS_ID 48 bits Applicable only whenHO_IND_type is set to 00 HMAC Tuple 21 bytes See 11.4.11 }

As shown in Table 10, the MOB_HO_IND message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, the ‘HO_IND_type’ representing a determination,cancellation or rejection for handover to the final target BS selectedby the MSS, the ‘Target_BS_ID’ representing an ID of the target BSselected by the MSS when the handover is determined, and the ‘FMACTuple’ for authenticating the MOB_HO_IND message. In the ‘HO_IND_type’,when the MSS has determined to perform the handover to the final targetBS, the MSS transmits the MOB_HO_IND message in which the ‘HO_IND_type’has been set to 00. When the MSS has determined to cancel the handoverto the final target BS, the MSS transmits the MOB_HO_IND message inwhich the ‘HO_IND_type’ has been set to 01. When the MSS has determinedto reject the handover to the final target BS, the MSS transmits theMOB_HO_IND message in which the ‘HO_IND_type’ has been set to 10. Theserving BS 210 having received the MOB_HO_IND message in which the‘HO_IND_type’ has been set to 10 creates a new the handover-executabletarget BS list and then retransmits the MOB_BSHO_RSP message to the MSS200.

Further, the serving BS 210 having received the MOB_HO_IND message inwhich the ‘HO_IND_type’ has been set to 00 recognizes that the MSS 200is to be handed over to the target BS (i.e., second target BS 230)contained in the MOB_HO_IND message. Then, the serving BS 210 releasesthe connection information currently set with the MSS 200 or retains theconnection information set with the MSS 200 over a preset time until anotification representing the complete end of the handover procedure isreceived from the target BS (i.e., second target BS 230) finallyselected by the MSS 200 (step 239). In this way, after the MSS 200 hastransmitted the MOB_HO_IND message to the serving BS 210, the MSS 200performs a remaining handover operation with the second target BS 230.

FIG. 3 is a flow diagram illustrating a general handover process by therequest of the BS in the IEEE 802.16e communication system.

Before a description about FIG. 3 is given, the handover process by therequest of the BS occurs when the BS is overloaded and requires a loadsharing for dispersing the load of the BS to neighbor BSs, or the BSmust cope with the change of the uplink status of an MSS. Referring toFIG. 3, a serving BS 310 transmits a MOB_NBR_ADV message to an MSS 300(step 311). The MSS 300 may obtain information on the neighbor BSs byreceiving the MOB_NBR_ADV message.

When the serving BS 310 detects the need for a handover of the MSS 300controlled by the serving BS 310 (step 313), the serving BS 310transmits the HO_PRE_NOTIFICATION messages to the neighbor BSs (steps315 and 317). Herein, the HO-PRE-NOTIFICATION message includesinformation for the bandwidth and the service level which must beprovided to the MSS 300 from a target BS which is to be a new serving BSof the MSS 300. In FIG. 3, it is assumed that the neighbor BSs of theserving BS 310 are a first target BS 320 and a second target BS 330.

After receiving the HO_PRE_NOTIFICATION messages, each of the firsttarget BS 320 and the second target BS 330 transmit theHO_PRE_NOTIFICATION_RESPONSE message to the serving BS 310 as a responsemessage of the HO_PRE_NOTIFICATION message (steps 319 and 321). TheHO_PRE_NOTIFICATION_RESPONSE message includes the ACK/NACK, whichrepresents if the target BSs can perform the handover requested by theserving BS 310, and the bandwidth and service level information capableof being provided to the MSS 300 as described in Table 7.

After the serving BS 310 receives the HO_PRE_NOTIFICATION_RESPONSEmessages from the first target BS 320 and the second target BS 330, theserving BS 310 selects the target BSs capable of providing the bandwidthand service level requested by the MSS 300. For example, when it isassumed that the service level capable of being provided by the firsttarget BS 320 is less than that requested by the MSS 300, and theservice level capable of being provided by the second target BS 330 isequal to that requested by the MSS 300, the serving BS 310 selects thesecond target BS 330 as a final target BS to which the MSS 300 can behanded over. After selecting the second target BS 330 as the target BSto which the MSS 300 can be handed over, the serving BS 310 transmits aMobile BS HandOver Request (MOB_BSHO_REQ) message including thehandover-executable target BS list to the MSS 300 (step 323). Thehandover-executable target BS list may include multiple target BSs. TheMOB_BSHO_REQ message has a structure as shown in Table 11. TABLE 11Syntax Size Notes MOB_BSHO_REQ_message_Format( ) { Management MessageType=51 8 bits For (j=0;j< N_Recommended N_Recommended;J++){ can bederived from the known length of the message Neighbor BS-ID 48 bits Service level predic- 8 bits tion } }

As shown in Table 11, the MOB_BSHO_REQ message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, and information on the target BSs selected by theserving BS 310. In table 11, the ‘N_Recommended’ represents the numberof neighbor BSs selected as the target BSs to which the MSS 300 can behanded over by the serving BS 310. Further, the MOB_BSHO_REQ messageincludes the IDs for each neighbor BS, which are represented by the‘N_Recommended’, and the information on the bandwidth and service levelcapable of being provided to the MSS 300 from the neighbor BSs.

The MSS 300 having received the MOB_BSHO_REQ message recognizes that thehandover has been requested by the serving BS 310, and selects a finaltarget BS to which the MSS 300 is to be handed over with reference tothe ‘N_Recommended’ information contained in the MOB_BSHO_REQ message.Before selecting the final target BS, when the MSS 300 intends to scanthe CINRs of the pilot channel signals transmitted from the serving BS310 and the neighbor BSs, the MSS 300 transmits the MOB_SCN_REQ messageto the serving BS 310 (step 325). Herein, since a point in time at whichthe MSS 300 requests the scan has no direct connection to the scanningoperation for the CINRs of the pilot channel signals, the detaileddescription will be omitted. The serving BS 310 having received theMOB_SCN_REQ message transmits the MOB_SCN_RSP message includinginformation to be scanned by the MSS 300 to the MSS 300 (step 327). TheMSS 300 having received the MOB_SCN_RSP message including the scanninginformation scans the CINRs of the pilot channel signals for theneighbor BSs, which have been recognized through the reception of theMOB_NBR_ADV message, the target BSs to which the MSS 300 can be handedover, which have been recognized through the reception of theMOB_BSHO_REQ message, and the serving BS 310 according to the parameters(i.e., Duration) included in the MOB_SCN_RSP message (step 329).

After selecting the final target BS to which the MSS 300 is to be handedover, the MSS 300 transmits the MOB_MSSHO_RSP message the serving BS 310as a response message for the MOB_BSHO_REQ message (step 331). TheMOB_MSSHO_RSP message has a structure as shown in Table 12. TABLE 12Syntax Size Notes MOB_MSSHO_RSP_message_Format( ) { Management MessageType=54 8 bits Estimated HO Time 8 bits For (j=0;j< N_Recommended;J++){N_Recommended can be derived from the known length of the messageNeighbor BS-ID 48 bits  BS S/(N+1) 8 bits } }

As shown in Table 12, the MOB_MSSHO_RSP message includes a plurality ofIEs, that is, the ‘Management Message Type’ representing the type of atransmitted message, the Estimated HO Time predicted as a start time ofthe handover procedure, and information on the target BSs selected bythe MSS. In table 12, the ‘N_Recommended’ represents the number of theneighbor BSs selected as the target BSs to which the MSS can be handedover. Further, the MOB_MSSHO_RSP message includes the IDs for eachneighbor BS, which are represented by the ‘N_Recommended’, and theinformation on the service level capable of being provided to the MSSfrom the neighbor BSs.

The serving BS 310 transmits the HO_CONFIRM message to the neighbor BSselected as the final target BS by the MSS 300 in response to theHO_PRE_NOTIFICATION-RESPONSE message (step 333). The MSS 300 havingselected the final target BS transmits the MOB_HO_IND message in whichthe ‘HO_IND_type’ has been set to 00 to the serving BS 310 (step 335).After receiving the MOB_HO_IND message, the serving BS 310 recognizesagain that the MSS 300 is to be handed over to the final target BSincluded in the MOB_HO_IND message. Then, the serving BS 310 releasesconnection information currently set with the MSS 300, or retains theconnection information set with the MSS 300 during a preset time until anotification representing the completion of the handover procedure isreceived from the target BS (i.e., second target BS 330) finallyselected by the MSS 200 (step 337). In this way, after the MSS 300 hastransmitted the MOB_HO_IND message to the serving BS 310, the MSS 300performs a remaining handover operation with the second target BS 330.

FIG. 4 is a flow diagram illustrating a general network reentry processaccording to the handover of the MSS in the IEEE 802.16e communicationsystem.

Referring to FIG. 4, an MSS 400 changes the connection to a final targetBS 450 and acquires the downlink synchronization with the final targetBS 450. Then, the MSS 400 receives a DownLink_MAP (DL_MAP) messagetransmitted from the final target BS 450 (step 411). The DL_MAP messageincludes parameters relating to a downlink of the final target BS 450.The MSS 400 receives an UpLink_MAP (UL_MAP) message transmitted from thefinal target BS 450 (step 413). The UL_MAP message is a messageincluding parameters relating to an uplink of the final target BS 450.Further, the UL_MAP message includes an FAST_UL_RANGING_IE which thefinal target BS 450 has assigned to support an FAST_UL_RANGING of theMSS 400 performing the handover. The reason for the final target BS 450to assign the FAST_UL_RANGING IE to the MSS 400 is to minimize a delaywhich may occur when the MSS 400 performs the handover. Accordingly, theMSS 400 may perform an initial ranging with the final target BS 450 in acontention-free scheme according to the FAST_UL_RANGING_IE. Herein, theFAST UL RANGING IE included in the UL_MAP message is as shown in Table13. TABLE 13 Syntax Size Notes Fast_UL_ranging_IE { MAC address 48 bitsMSS MAC address as provided on the RNG_REQ message on initial systementry. UIUC 4 bits UIUC = 15. A four-bit code used to define the type ofuplink access and the burst type associated with that access. OFDMSymbol offset 10 bits The offset of the OFDM symbol in which the burststarts, the offset value is defined in units of OFDM symbols and isrelevant to the Association Start Time field given in the UL_MAPmessage. Subchannel offset 6 bits The lowest index OFDMA subchannel usedfor carrying the burst, starting from subchannel 0. No OFDM Symbol 10bits The number of OFDM symbols that are used to carry the UL Burst. NoSubchannels 6 bits The number of OFDMA subchannels with subsequentindexes, used to carry the burst. Reserved 4 bits }

The FAST_UL_RANGING_IE in Table 13 includes information for the MACaddress of an MSS which is to obtain a ranging opportunity, the UplinkInterval Usage Code (UIUC) for providing region information including astart offset value for the FAST_UL_RANGING, and the offset of a rangingopportunity interval of a contention-free scheme/the number ofsymbols/the number of subchannels, which are assigned to the MSS 400,etc. The MAC address of the MSS 400 is notified to the final target BS450 through a message such as the HO_PRE_NOTIFICATION message, theHO_PRE_NOTIFICATION_RESPONSE message and the HO_CONFIRM messageexchanged between the serving BS and the final target BS in the backbonenetwork in the handover process described in FIGS. 2 and 3.

The MSS 400 having received the UL_MAP message transmits a RangingRequest (RNG_REQ) message to the final target BS 450 according to theFAST_UL_RANGING_IE (step 415). The final target BS 450 having receivedthe RNG_REQ message transmits a Ranging Response (RNG_RSP) messageincluding the information for compensating for a frequency, a time and atransmit power for the ranging to the MSS 400 (step 417).

The MSS 400 and the final target BS 450 having completed the initialranging perform a re-authorization operation (MSS RE_AUTHORIZATION) forthe MSS 400 (step 419). In the performance of the re-authorizationoperation, when a security context exchanged between a serving BS towhich the MSS 400 previously belongs and the final target BS 450 has notchanged, the final target BS 450 uses the security context as is. Abackbone network message for providing the security context informationof the MSS 400, that is, an MSS-information-response (MSS_INFO_RSP)message has a structure as shown in Table 14. TABLE 14 Field Size NotesGlobal Header 152-bit For (j=0;j< Num_Records;J++){ MSS uniqueidentifier  48-bit 48-bit unique iden- tifier used by MSS (as providedby the MSS or by the I-am- host-of message) N_NSIE Number of NetworkService Information Elements For (k=0;k< N_NSIE;k++){ Field Size  16-bitSize of TLV encoded information field below TLV encoded infor- VariableTLV information as mation followed on a DSA- REQ MAC message } N_SAIENumber of Security Association Infor- mation Elements For (k=0;k<N_SAIE;k++){ Field Size  16-bit Size of TLV encoded information fieldbelow TLV encoded infor- Variable TLV information as mation followed ona PKM- xxx MAC message } N_MSS_CAP Number of MSS Capa- bilities For(k=0;k<N_MSS_CAP; k++){ Field Size  16-bit Size of TLV encodedinformation field below TLV encoded infor- Variable TLV information asmation followed on a SBC- REQ MAC message } TLV encoded infor- VariableTLV information as mation followed on a SBC- REQ MAC message } Securityfield TBD A means to authenti- cate this message CRC field  32-bit IEEECRC-32

As shown in Table 14, the MSS_INFO_RSP message includes the IDinformation of MSSs registered in the serving BS, the security contextinformation such as Security Association Information for each MSS, theNetwork Service Information for each MSS, the capability information ofeach MSS, etc.

When the re-authorization operation has been completed for the finaltarget BS 450 and the MSS 400, the MSS 400 transmits a RegistrationRequest (REG_REQ) message to the final target BS 450 (step 421). TheREG_REQ message includes the registration information of the MSS 400.The final target BS 450 transmits a Registration Response (REG_RSP)message to the MSS 400 in response to the REG_REQ message (step 423).The final target BS 450 detects the registration information of the MSS400 contained in the received REG_REQ message, thereby recognizing thatthe MSS 400 is an MSS having performed the handover. Accordingly, thefinal target BS 450 maps the connection setup information of the MSS 400in the previous serving BS with the connection setup information of theMSS 400 in the final target BS 450. Further, the final target BS 450inserts a TLV value, which is used for resetting a service flow capableof being actually received, into the MSS_INFO_RSP message and thentransmits the MSS_INFO_RSP message to the MSS 400. Table 15 shows thestructure of the TLV including mapping information for connection setupin the serving BS and the final target BS 450. TABLE 15 Type Length Name(1 byte) (1 byte) Value (Variable-length) New_CID 2.1 2 New CID afterhand-over to new BS Old_CID 2.2 2 Old CID before hand-over from old BSConnection 2.3 Variable If any of the service flow para- Info meterschange, then those service flow parameters and CS parameter encodingTLVs that have changed will be added. Connection_Info is a compound TLVvalue that encapsulates the Service Flow Parameters and the CS parameterthat have changed for the service. All the rules and settings that applyto the parameters when used in the DSC-RSP message apply to the contentsencapsulated in this TLV.

In Table 15, the TLV contained in the REG_RSP message provides a CIDused in the serving BS before the MSS 400 performs the handover, and theCID information which is to be used in the final target BS 450 after theMSS 400 has performed the handover. Further, when the final target BS450 provides a service different from a service flow provided from theserving BS before the handover, the TLV includes information for anychanged service parameters.

The MSS 400 completes the network reentry procedure with the finaltarget BS 450 and performs a normal communication service through thefinal target BS 450 (step 425).

SUMMARY OF THE INVENTION

As described above, when the CINR of the pilot signal of the serving BSis reduced to the extent that communication with the current serving BScannot be continued in the IEEE 802.16e communication system, the MSS ishanded over to a neighbor BS (i.e., final target BS) that is differentfrom the serving BS according to the request of the MSS or the requestof a BS. However, while the MSS performs the network reentry operationwith the final target BS in the IEEE 802.16e communication system, whenthe CINR of the pilot signal transmitted from the final target BS isreduced and a communication service through the final target BS isimpossible, the MSS can change the connection to the serving BS.

Further, after the MSS has changed the connection to the serving BS dueto a pingpong effect occurred during the handover to the final targetBS, the MSS must carry out an initial connection setup procedure withthe serving BS, that is, the network reentry operation, in order toreopen a communication service through the serving BS. Accordingly, whenthe pingpong effect frequently occurs during the handover by the MSS,the MSS must frequently carry out the network reentry operation.Therefore, a service may be delayed. Further, the frequent execution ofthe network reentry operation increases a signaling load, therebydeteriorating the entire performance of the system.

Accordingly, the present invention has been made to solve at least theabove-mentioned problems occurring in the prior art, and it is an objectof the present invention is to provide a system and a method forperforming a handover to prevent a pingpong effect by recognizing inadvance the occurrence of the pingpong effect during the handover in aBWA communication system.

It is another object of the present invention is to provide a system anda method for performing a handover to minimize a communication servicedelay when a pingpong effect occurs in a BWA communication system.

In order to accomplish the aforementioned object, according to oneaspect of the present invention, there is provided a method forperforming a handover by a Mobile Subscriber Station (MSS) in aBroadband Wireless Access (BWA) communication system including a servingBase Station (BS) for providing a service to the MSS and a target BSadjacent to the serving BS. The method comprises the steps of receivinga handover request message from the serving BS, the handover requestmessage including a resource remain type field representing whetherconnection information having been set between the serving BS and theMSS is retained and recognizing whether the serving BS retains theconnection information according to a value of the resource remain typefield.

In order to accomplish the aforementioned object, according to anotheraspect of the present invention, there is provided a method forperforming a handover by a Mobile Subscriber Station (MSS) in aBroadband Wireless Access (BWA) communication system including a servingBase Station (BS) for providing a service to the MSS and a target BSadjacent to the serving BS. The method comprises the steps of requestingthe handover to the serving BS; receiving a handover response messagefrom the serving BS, the handover response message including a resourceremain type field representing whether connection information havingbeen set between the serving BS and the MSS is retained; and recognizingwhether the serving BS retains the connection information according to avalue of the resource remain type field.

In order to accomplish the aforementioned object, according to a furtheraspect of the present invention, there is provided a method forperforming a handover by a serving Base Station (BS) for providing aservice to a Mobile Subscriber Station (MSS) in a Broadband WirelessAccess (BWA) communication system including a target BS adjacent to theserving BS. The method comprises the steps of transmitting a handoverrequest message to the MSS, the handover request message including aresource remain type field representing whether connection informationhaving been set between the serving BS and the MSS is retained;receiving a handover indication message from the MSS; and retaining theconnection information according to a value of the resource remain typefield.

In order to accomplish the aforementioned object, according to stillanother aspect of the present invention, there is provided a method forperforming a handover by a serving Base Station (BS) for providing aservice to a Mobile Subscriber Station (MSS) in a Broadband WirelessAccess (BWA) communication system including a target BS adjacent to theserving BS. The method comprises the steps of receiving a handoverrequest message from the MSS; transmitting a handover response messageto the MSS, the handover response message including a resource remaintype field representing whether connection information having been setwith the MSS is retained; receiving a handover indication message fromthe MSS; and retaining the connection information according to a valueof the resource remain type field.

In order to accomplish the aforementioned object, according to stillanother aspect of the present invention, there is provided a handovermethod for minimizing a service delay due to a pingpong effect in aBroadband Wireless Access (BWA) communication system including a MobileSubscriber Station (MSS), a serving Base Station (BS) communicating withthe MSS, and a plurality of neighbor BSs. The method comprises the stepsof a) requesting to the serving BS a handover when it is detected thathandover must be performed from the serving BS to a final target BSwhich is one of the neighbor BSs; b) detecting information for at leasttwo candidate target BSs, on of which is selected as the final targetBS, according to the handover request, the information being receivedfrom the serving BS; c) measuring CINR (Carrier to Interference andNoise Ratio) values transmitted from each of the candidate target BSs;d) comparing the measured CINR values with a first predeterminedthreshold value and excluding a first candidate target BS, whichtransmits a reference signal having a CINR value less than the firstthreshold value, from the candidate target BSs when CINR values lessthan the first threshold value exist; and e) determining the finaltarget BS in the candidate target BSs excluding the first candidatetarget BS and notifying the serving BS of a handover to the final targetBS.

In order to accomplish the aforementioned object, according to yetanother aspect of the present invention, there is provided a handovermethod for minimizing a service delay due to a pingpong effect in aBroadband Wireless Access (BWA) communication system including a MobileSubscriber Station (MSS), a serving Base Station (BS) communicating withthe MSS, and a plurality of neighbor BSs. The method comprising thesteps of a) receiving a handover request including information for atleast two candidate target BSs from the serving BS, one of the candidatetarget BSs being selected as a final target BS by the serving BS, thefinal target BS being one of the neighbor BSs, and detecting theinformation; b) measuring CINR (Carrier to Interference and Noise Ratio)values transmitted from each of the candidate target BSs; c) comparingthe measured CINR values with a first predetermined threshold value andexcluding a first candidate target BS, which transmits a referencesignal having the CINR value less than the first threshold value, fromthe candidate target BSs when CINR values less than the first thresholdvalue exist; and d) determining the final target BS in the candidatetarget BSs excluding the first candidate target BS and notifying theserving BS of handover to the final target BS.

In order to accomplish the aforementioned object, according to yetanother aspect of the present invention, there is provided a handovermethod for minimizing a service delay due to a pingpong effect in aBroadband Wireless Access (BWA) communication system including a MobileSubscriber Station (MSS), a serving Base Station (BS) communicating withthe MSS, and a plurality of neighbor BSs. The method comprises the stepsof a) one of deleting connection information of the MSS and retainingthe connection information during a preset time, when the serving BSreceives a notification representing that the MSS is to be handed overto a target BS which is a specific neighbor BS of the neighbor BSs; b)changing a connection to the serving BS when the MSS recognizes anoccurrence of a pingpong effect during a network reentry procedure withthe target BS; c) receiving one of a handover request message and ahandover response message from the serving BS; d) detecting andrecognizing if the connection information for the MSS is retained, whichis contained in each message, according to reception of each message;and e) performing only an initial ranging procedure and reopeningcommunication with the serving BS when the serving BS retains theconnection information for the MSS.

In order to accomplish the aforementioned object, according to yetanother aspect of the present invention, there is provided a handoversystem for minimizing a service delay due to a pingpong effect in aBroadband Wireless Access (BWA) communication system including an MSS(Mobile Subscriber Station), a serving BS (Base Station) forcommunicating with the MSS, and a plurality of neighbor BSs. The systemcomprises a MSS for requesting handover to a serving BS when it isdetected that handover must be performed from the serving BS to a finaltarget BS which is one of the neighbor BSs, detecting information for atleast two candidate target BSs, which are selected as the final targetBS, according to the handover request, the information being receivedfrom the serving BS, measuring CINR (Carrier to Interference and NoiseRatio) values transmitted from each of the candidate target BSs,comparing the measured CINR values with a first predetermined thresholdvalue and excluding a first candidate target BS, which transmits areference signal having a CINR value less than the first thresholdvalue, from the candidate target BSs when CINR values less than thefirst threshold value exist, and determining the final target BS in thecandidate target BSs excluding the first candidate target BS andnotifying the serving BS of handover to the final target BS; and aserving BS for receiving the request of the MSS, transmitting theinformation for at least two candidate target BSs, which are selected asthe final target BS by the MSS, and receiving a notificationrepresenting that the MSS is to be handed over to the final target BSfrom the MSS.

In order to accomplish the aforementioned object, according to yetanother aspect of the present invention, there is provided a handoversystem for minimizing a service delay due to a pingpong effect in aBroadband Wireless Access (BWA) communication system including an MSS(Mobile Subscriber Station), a serving BS (Base Station) forcommunicating with the MSS, and a plurality of neighbor BSs. The systemcomprises a MSS for receiving a handover request including informationfor at least two candidate target BSs from a serving BS, the candidatetarget BSs being selected as a final target BS by the serving BS, thefinal target BS being one of the neighbor BSs, and detecting theinformation, measuring CINR (Carrier to Interference and Noise Ratio)values transmitted from each of the candidate target BSs, comparing themeasured CINR values with a first predetermined threshold value andexcluding a first candidate target BS, which transmits a referencesignal having the CINR value less than the first threshold value, fromthe candidate target BSs when CINR values less than the first thresholdvalue exist, and determining the final target BS in the candidate targetBSs excluding the first candidate target BS and notifying the serving BSof handover to the final target BS; and a serving BS for requesting thehandover of the MSS and receiving information for the final target BSfrom the MSS.

In order to accomplish the aforementioned object, according to yetanother aspect of the present invention, there is provided a system forperforming a handover in a Broadband Wireless Access (BWA) communicationsystem including a Mobile Subscriber Station (MSS), a serving BaseStation (BS) for providing a service to the MSS, and a target BSadjacent to the serving BS. The system comprises a MSS for requestingthe handover to the serving BS, receiving a handover response messagefrom the serving BS, the handover response message including a resourceremain type field representing whether preset connection information isretained, and recognizing whether the serving BS retains the connectioninformation according to a value of the resource remain type field; anda serving BS for receiving a handover request message from the MSS,transmitting the handover response message to the MSS, the handoverresponse message including the resource remain type field representingwhether the connection information having been set with the MSS isretained, receiving a handover indication message from the MSS, andretaining the connection information according to the value of theresource remain type field.

In order to accomplish the aforementioned object, according to yetanother aspect of the present invention, there is provided a system forperforming a handover in a Broadband Wireless Access (BWA) communicationsystem including a Mobile Subscriber Station (MSS), a serving BaseStation (BS) for providing a service to the MSS, and a target BSadjacent to the serving BS. The system comprises a MSS for receiving ahandover request message from the serving BS, the handover requestmessage including a resource remain type field representing whetherconnection information having been set with the serving BS is retained,and recognizing whether the serving BS retains the connectioninformation according to a value of the resource remain type field; anda serving BS for transmitting the handover request message to the MSS,the handover request message including the resource remain type fieldrepresenting whether the connection information having been set with theMSS is retained, receiving a handover indication message from the MSS,and retaining the connection information according to the value of theresource remain type field.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing the general structure of an IEEE802.16e communication system;

FIG. 2 is a flow diagram illustrating a general handover process by therequest of an MSS in an IEEE 802.16e communication system;

FIG. 3 is a flow diagram illustrating a general handover process by therequest of a BS in an IEEE 802.16e communication system;

FIG. 4 is a flow diagram illustrating a general network reentry processaccording to handover of an MSS in an IEEE 802.16e communication system;

FIG. 5 is a flow diagram illustrating an operation process of an MSSwhen a pingpong effect occurs during a handover in an IEEE 802.16ecommunication system according to an embodiment of the presentinvention;

FIG. 6 is a flow diagram illustrating an operation process of an MSSaccording to whether the MSS recognizes the occurrence of an pingpongeffect when a handover situation occurs in an IEEE 802.16e communicationsystem according to an embodiment of the present invention;

FIG. 7 is a diagram illustrating a relation between CINR valuesaccording to BSs, which allow an MSS to recognize a pingpong effect, inan IEEE 802.16e communication system according to an embodiment of thepresent invention;

FIG. 8 is a flow diagram illustrating an operation process of an MSSwhen a pingpong effect occurs during the performance of a networkreentry procedure according to handover in an IEEE 802.16e communicationsystem according to an embodiment of the present invention;

FIG. 9 is a diagram illustrating a relation between CINR valuesaccording to BSs, which are used for recognizing a pingpong effectoccurring during the performance of a network reentry procedureaccording to handover in an IEEE 802.16e communication system accordingto an embodiment of the present invention;

FIG. 10 is a flow diagram illustrating a process by which an MSSrecognizes whether a serving BS retains connection information in anIEEE 802.16e communication system according to an embodiment of thepresent invention; and

FIG. 11 is a flow diagram illustrating a handover operation process ofan MSS that recognizes whether a serving BS retains connectioninformation of the MSS in an IEEE 802.16e communication system accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a preferred embodiment according to the present inventionwill be described with reference to the accompanying drawings. In thefollowing description of the present invention, a detailed descriptionof known functions and configurations incorporated herein will beomitted when it may obscure make the subject matter of the presentinvention.

The present invention proposes a scheme for recognizing the occurrenceof a pingpong effect and minimizing a service delay due to the pingpongeffect while an MSS performs a handover operation with a target BS towhich the MSS is to be handed over in an IEEE 802.16e communicationsystem which is a BWA communication system. The present inventionproposes a scheme for recognizing the occurrence of the pingpong effectand preventing the pingpong effect when the MSS cancels the handover tothe target BS during performance of the handover operation with thetarget BS and then changes the connection to a serving BS. The presentinvention proposes a scheme for allowing the serving BS to retainconnection information of the MSS according to channel conditions, sothat the MSS can quickly perform a connection setup according to thehandover. The IEEE 802.16e communication system is a BWA communicationsystem using an OFDM scheme and an OFDMA scheme. The IEEE 802.16ecommunication system using the OFDM/OFDMA scheme transmits physicalchannel signals by means of multiple sub-carriers, thereby allowing datato be transmitted at a high speed. Further, the IEEE 802.16ecommunication system is a communication system supporting the mobilityof the MSS by supporting a multi-cell structure.

FIG. 5 is a flow diagram illustrating an operation process of the MSSwhen the pingpong effect occurs during a handover in the IEEE 802.16ecommunication system according to an embodiment of the presentinvention.

Referring to FIG. 5, in step 502, the MSS performs a message exchangeprocedure relating to a series of handover requests and responses. Thatis, the MSS transmits a MOB_MSSHO_REQ message to a BS (i.e., serving BS)providing a service to the MSS and then receives a MOB_BSHO_RSP messageas a response for the MOB_MSSHO_REQ message. Also, the MSS receives aMOB_BSHO_REQ message from the serving BS and then transmits aMOB_MSSHO_RSP message as a response for the MOB_BSHO_REQ message. Then,step 504 is performed. In step 504, the MSS detects and recognizeshandover-executable final target BS candidate list information containedin the handover request or response message transmitted/received to/fromthe serving BS. Then, step 506 is performed. In step 506, the MSSmeasures a CINR for each handover-executable final target BS candidate.Then, step 508 is performed. In step 508, the MSS determines aMOB_HO_IND_type according to whether the MSS recognizes the occurrenceof a pingpong effect which will be described later. Then, step 510 isperformed. In step 510, the MSS transmits a MOB_HO_IND message includinginformation regarding the determined type to the serving BS.

FIG. 6 is a flow diagram illustrating an operation process of the MSSaccording to whether the MSS recognizes the occurrence of the pingpongeffect when a handover situation occurs in an IEEE 802.16e communicationsystem according to an embodiment of the present invention.

Referring to FIG. 6, in step 602, when the MSS recognizes the occurrenceof a situation in which the handover must be performed, the MSS sets acounter initial value i for neighbor BSs to 0 and sets a HandOver cancelflag (HO_CANCEL_FLAG) to 0. Then, step 604 is performed. That is, theMSS performs an initialization procedure for measuring the CINR valuesfor each of the neighbor BS contained in the handover-executable finaltarget BS candidate list. The HO_CANCEL_FLAG is a flag for reportingthat remaining neighbor BSs except for a serving BS from among theneighbor BSs contained in the handover-executable final target BScandidate list that have not been selected as final target BSs. In step604, the MSS determines if the CINR values have been measured for all ofthe BSs in the final target BS candidate list satisfying thepredetermined conditions from among the neighbor BSs aligned in asequence having better factors synthetically reflecting a QoS, a CINR,etc. As a result of the determination, when there exist BSs for whichthe CINR values has not been measured, that is, i<N_Neighbors, step 606is performed.

In step 606, the MSS examines whether the CINR value (CINR_BS) is lessthan a preset pingpong threshold value (PP_THRESHOLD) from thehighly-ranked BS from among the aligned final target BS candidates.Herein, the PP_THRESHOLD is set to have a value greater than a handoverthreshold value (HO_THRESHOLD) which is a threshold value used todetermine that the MSS must be handed over from the serving BS includingthe MSS to another BS other than the serving BS. In other words, thePP_THRESHOLD is set to allow the MSS to be handed over to a reliabletarget BS. The PP_THRESHOLD is set to prevent an unnecessary handoverdue to the pingpong effect. As a result of the examination, when theCINR_BS is less than the PP_THRESHOLD, step 608 is performed.

In step 608, the MSS deletes the neighbor BS from the final target BScandidate list because the CINR_BS is less than the PP_THRESHOLD, andstep 610 is performed. In step 610, the MSS increases the i value by 1and repeats the steps from step 604 for BSs in the next sequence on thelist. If the CINR of the neighbor BS is greater than or equal to thePP_THRESHOLD in step 606, that is, the CINR_BS≧ the PP_THRESHOLD, step612 is performed. In step 612, the MSS determines if the BS is theneighbor BS or the serving BS. As a result of the determination, whenthe BS is the serving BS, the MSS sets in step 614 the HO_CANCEL_FLAG toa value of 1, and step 610 is performed. In contrast, when the BS is theneighbor BS, step 616 is performed. In step 616, the MSS determines ifthe HO_CANCEL_FLAG having been set up to now has a value of 1. As aresult of the determination, when the HO_CANCEL_FLAG does not have avalue of 1, step 618 is performed. In contrast, when the HO_CANCEL_FLAGhas a value of 1, step 620 is performed. In step 618, the MSS determinesthat the BS is a final target BS to which the MSS is to be handed over,and step 622 is performed. In step 622, the MSS transmits aMOB_HO_IND_type message (type=00) to the current serving BS so as toreport the handover to the determined final target BS. In step 620, theMSS sets the HO_CANCEL_FLAG to have a value of 0 again and determinesthe BS as the final target BS. Then, step 622 is performed.

As a result of the checking in step 606 based on the measurment of theCINRS for all of the BSs in the final target candidate list includingthe serving BS, when the MSS has not found a BS having a CINR valueexceeding the PP_THRESHOLD, the process proceeds to step 624 from step604. In step 624, the MSS determines if the HO_CANCEL_FLAG is set to 1.As a result of the determination, when the HO_CANCEL_FLAG has been setto 1, step 626 is performed. In step 626, the MSS transmits a HO_INDmessage in which the HO_IND_type has been set to 01, that is, the HO_INDmessage including handover cancellation (HO_CANCEL) information, to theserving BS. Therefore, the MSS notifies the serving BS of a handovercancellation. In step 624, when the HO_CANCEL_FLAG has not been set to1, step 628 is performed. That is, in step 628, the MSS transmits aHO_IND message in which the HO_IND_type is set to 01, that is, theHO_IND message including handover rejection (HO_REJECT) information, tothe serving BS. Therefore, the MSS notifies the serving BS of a handoverrejection.

As described above, since the MSS aligns the neighbor BSs contained inthe handover-executable final target BS candidate list from a BS havingan optimal handover condition, when the MSS selects one of the remainingneighbor BSs except for the serving BS as a final target BS through theprocedure regarding whether the occurrence of the pingpong effect isrecognized, the selected target BS corresponds to a BS having favorableconditions as compared with other neighbor BSs for which the procedureregarding whether the occurrence of the pingpong effect is recognizedhas not been performed. Accordingly, the MSS transmits theMOB_HO_IND_type message including a HO_RELEASE option to the serving BS,so that the MSS can determine handover to the neighbor BS selected asthe final target BS.

FIG. 7 is a diagram illustrating a relation between CINR valuesaccording to BSs, which allow the MSS to recognize a pingpong effect, inthe IEEE 802.16e communication system according to an embodiment of thepresent invention.

Referring to FIG. 7, the CINR values and threshold values required forallowing the MSS to recognize the pingpong effect are described. ThePP_THRESHOLD 700 represents a threshold value in CINR values proper forreceiving a normal communication service after the handover, and theHO_THRESHOLD 750 represents a threshold value in CINR values preset bythe MSS in order to select a handover-executable target BS. Further, thereference number 710 represents the CINR value of a serving BS, thereference number 720 represents the CINR value of a first neighbor BS,and the reference number 730 represents the CINR value of a secondneighbor BS. That is, when a CINR value having been measured for arandom BS by the MSS is less than the PP_THRESHOLD and greater than theHO_THRESHOLD, the pingpong effect may occur.

The relation between the CINR values according to BSs shown in FIG. 7will be described. In the reference number 702, since the CINR value 710of the serving BS measured by the MSS is less than the HO_THRESHOLD 750,and the CINR value 720 of the first neighbor BS and the CINR value 730of the second neighbor BS are greater than the CINR value 710 of theserving BS, the first neighbor BS and the second neighbor BS may beselected as handover-executable target BSs. However, since the CINRvalue 720 of the first neighbor BS and the CINR value 730 of the secondneighbor BS are less than the PP_THRESHOLD, the first neighbor BS andthe second neighbor BS are not selected as final target BSs. Herein,while the MSS ignores the PP_THRESHOLD and is handed over to the secondneighbor BS, when the CINR value 730 of the second neighbor BS ismeasured again, it is highly possible that the newly measured the CINRvalue 730 may be less than the HO_THRESHOLD. This implies the occurrenceof the pingpong effect which may require handover of the MSS to anotherBS again.

In the reference number 704, the MSS can understand that the CINR value710 of the serving BS and the CINR value 720 of the first neighbor BSare greater than the HO_THRESHOLD, and the CINR value 730 of the secondneighbor BS is less than the PP_THRESHOLD. Since there is no BS having aCINR value greater than the PP_THRESHOLD even in the reference number704, a final target BS does not exist.

In the reference number 706, the MSS can understand that the CINR value720 of the first neighbor BS is less than the CINR value 710 of theserving BS which is less than the CINR value 730 of the second neighborBS, and the three ,CINR values are greater than the HO_THRESHOLD. Evenin this case, since there is no BS having a CINR value greater than thePP_THRESHOLD, the final target BS does not exist, similarly to thereference number 704.

In the reference number 708, the MSS can understand that the CINR value720 of the first neighbor BS is greater than the PP_THRESHOLD, the CINRvalue 710 of the serving BS and the CINR value 730 of the secondneighbor BS are less than the PP_THRESHOLD and greater than theHO_THRESHOLD. Accordingly, the MSS can select the first neighbor BS asthe final target BS to which the MSS is to be handed over.

FIG. 8 is a flow diagram illustrating an operation process of the MSSwhen the pingpong effect occurs during the performance of a networkreentry procedure according to the handover in the IEEE 802.16ecommunication system according to an embodiment of the presentinvention.

Referring to FIG. 8, in step 802, the MSS performs the network reentryprocedure with the final target BS through a series of proceduresrelating to the handover of FIGS. 2 and 3. In the course of performingthe network reentry procedure, step 804 is performed. In step 804, theMSS measures the CINR value of the final target BS, and step 806 isperformed. In step 806, the MSS compares the measured CINR value (i.e.,CINR_BS) of the final target BS with the PP_THRESHOLD. As a result ofthe comparison, when the CINR_BS is greater than the PP_THRESHOLD, step802 is performed to allow the network reentry procedure with the finaltarget BS to be continued. In contrast, when the CINR_BS is less thanthe PP_THRESHOLD in step 806, step 808 is performed. In step 808, theMSS recognizes that the pingpong effect may occur because the CINR_BS isless than the PP_THRESHOLD, and performs the process of FIG. 6corresponding this case.

FIG. 9 is a diagram illustrating a relation between CINR valuesaccording to BSs, which are used for recognizing the pingpong effectoccurring during the performance of the network reentry procedureaccording to the handover in the IEEE 802.16e communication systemaccording to an embodiment of the present invention.

Referring to FIG. 9, in the reference number 902, the CINR value 920 ofthe final target BS, which is measured by the MSS, is greater than thePP_THRESHOLD and the CINR value 910 of the serving BS is less than theHO_THRESHOLD. Accordingly, the MSS is handed over to the final targetBS. In the reference numbers 904 and 906, the CINR value 920 of thefinal target BS, which is measured by the MSS, is greater than thePP_THRESHOLD and the CINR value 910 of the serving BS is also greaterthan the HO_THRESHOLD. Accordingly, the MSS is handed over to the finaltarget BS. In the reference number 908, as a result of the measurementof the CINR values of the final target BS and the serving BS by the MSS,the two values are less than the PP_THRESHOLD and greater than theHO_THRESHOLD. Accordingly, the MSS recognizes that it is impossible forthe MSS to be handed over to the final target BS so as to perform acommunication, and the pingpong effect requiring handover of the MSS toanother BS may occur.

FIG. 10 is a flow diagram illustrating a process by which the MSSrecognizes whether the serving BS retains connection information in theIEEE 802.16e communication system according to an embodiment of thepresent invention.

Referring to FIG. 10, in step 1002, the MSS receives a MOB_BSHO_REQmessage or a MOB_BSHO_RSP message from the serving BS, and step 1004 isperformed. Herein, as described in the prior art, after the serving BShas received a MOB_HO_IND message (“HO_IND_Type=00”) transmitted fromthe MSS, the serving BS may delete the connection information with theMSS or retain the connection information for a predetermined time untilthe serving BS receives a notification representing the completion of ahandover procedure from the final target BS. If it is a case in whichthe serving BS has deleted the connection information with the MSS, theserving BS does not have the information for the MSS when the MSS haschanged a connection to the serving BS again due to the pingpong effectoccurred while the MSS performs the network reentry procedure with thefinal target BS. Accordingly, the serving BS and the MSS perform angeneral initial communication procedure. Further, if the MSS may be ableto determine if the serving BS has deleted the connection informationwith the MSS or retains the connection information for the predeterminedtime, the MSS performs the procedure corresponding to each case, so thatoverhead due to the pingpong effect can be reduced.

Further, the present invention provides a scheme by which the serving BShaving received the MOB_HO-IND message (“HO_IND_type=00”) from the MSSinforms the MSS that the serving BS will delete the connectioninformation of the MSS or retain the connection information for thepredetermined time as described above, thereby minimizing the servicedelay which may occur in a reconnection setup with the serving BS.Hereinafter, this scheme will be described with reference to Tables 16and 17.

Tables 16 and 17 show the formats of modified handover messages proposedby the present invention. The modified handover messages are obtained byadding information (Resource_Remain_Type) regarding retaining ordeletion of resource connection information for the MSS and informationregarding a resource retaining time (Resource_Retain_Time) to theMOB_BSHO_RSP message and the MOB_BSHO_REQ message contained in thehandover messages exchanged between the MSS and the serving BS when anexisting handover procedure is performed. TABLE 16 Syntax Size NotesMOB_BSHO-RSP_message_Format( ) { Management Message Type=?? 8 bitsEstimated HO time 8 bits For (j=0;j< N_Recommended;J++){ Neighbor basestations shall be presented in an order such that the first presented isthe one most recommended and the last presented is the leastrecommended. N_Recommended can be derived from the known length of themessage. Neighbor BS-ID 48 bits Service level prediction 8 bits }Resource_Remain_Type 1 bits 0: MSS resource release 1: MSS resourceretain Resource_Retain_Time 8 bits Time duration for case whereResource_Remain_Type value is 1. Units are frames. reserved 7 bitsReserved; shall be set to zero }

As shown in Table 16, the MOB_BSHO_RSP message newly proposed by thepresent invention has a structure obtained by newly adding theResource_Remain_Type field and the Resource_Retain_Time field to theexisting MOB_BSHO_RSP message. The Resource_Remain_Type field providesinformation regarding whether the serving BS will delete or retain theconnection information for the MSS. When the Resource_Remain_Type fieldshown in Table 16 has a value of 0, the serving BS deletes theconnection information for the MSS. In contrast, when theResource_Remain_Type field has a value of 1, the serving BS retains theconnection information for the MSS during a time of theResource_Retain_time field. The information for the predetermined timeduration is stored in the Resource_Retain_Time field by the frame. Forexample, when the MSS has received the MOB_BSHO_RSP message in which theResource_Remain_Type has been set to 1 and the Resource_Retain_Time hasbeen set to 10, the serving BS retains the connection information forthe MSS during 10 frames after receiving the MOB_HO_IND message(“HO_IND_type=00”). If the MSS has received the MOB_BSHO_RSP message inwhich the Resource_Remain_Type has been set to 1 and theResource_Retain_Time has been set to 0, the MSS recognizes that theserving BS retains the connection information of the MSS for a timeduration pre-negotiated during the performance of a registrationprocedure with the serving BS. Herein, the Resource_Retain_Time fieldhas a frame unit, and the frame unit has the same meaning as that of thetime unit. For example, 1 frame unit may be converted into a time unitof 20 ms.

When it is necessary to change a retaining time of the connectioninformation of the MSS, which has been pre-negotiated during theperformance of the registration procedure with the MSS, for example,when it is difficult to retain the connection information of the MSS,which is to be handed over to a target BS, during a preset time durationdue to the insufficiency of the buffer capacity of the serving BS, orwhen it is possible to retain the connection information of the MSS,which is to be handed over to the target BS, longer than the preset timeduration because the buffer conditions of the serving BS become betteras compared with the previous registration procedure performance, theserving BS may set the Resource_Retain_Time of the MOB_BSHO_RSP messageor the MOB_BSHO_REQ message to have a random value other than 0.

The following Table 17 shows the structure the MOB_BSHO_REQ messagenewly proposed by the present invention. TABLE 17 Syntax Size NotesMOB_BSHO_REQ_message_Format( ) { Management Message Type=?? 8 bits For(j=0;j< N_Recommended;J++){ N_Recommended can be derived from the knownlength of the message Neighbor BS-ID 48 bits Service level prediction 8bits } Resource_Remain_Type 1 bits 0: MSS resource release 1: MSSresource retain Resource_Retain_Time 8 bits Time duration for case whereResource_Remain_Type value is 1 reserved 7 bits Reserved; shall be setto zero }

As shown in Table 17, the MOB_BSHO_REQ message has a structure obtainedby adding the Resource_Remain_Type field and the Resource_Retain_Timefield to the existing MOB_BSHO_REQ message, similarly to theMOB_BSHO_RSP message. The Resource_Remain_Type field reports whether todelete the connection information or retain the connection informationduring a predetermined time duration, and the Resource_Retain_Time fieldreports the predetermined time duration when the connection informationis retained during the predetermined time duration. When the timepre-negotiated during the performance of the registration procedure withthe MSS is applied to the Resource_Retain_Time field similarly to theMOB_BSHO_RSP message, the serving BS sets the Resource_Retain_Time fieldto have a value of 0. Further, when the serving BS optionally determinesa connection information-retaining time, the serving BS sets theResource_Retain_Time field to have a random value other than 0.

When the serving BS retains the connection information of the handoveredMSS, the connection information may be usefully utilized not only inperforming handover due to the pingpong effect, but also when the MSScommunicates with a target BS requiring drop-recovery after finding thetarget BS requiring the drop-recovery in an occurrence of a dropsituation during a handover with the serving BS. That is, if the MSSrecognizes that the serving BS retains the connection information withthe MSS, the fact that the connection information of the MSS can bereceived from the serving BS through a backbone network is reported tothe drop recovery-targeted target BS, so that the drop recoveryprocedure can be performed within a short time.

Returning again to FIG. 10, in step 1004, the MSS performs the nextsteps according to values recorded in the Resource_Remain_Type fieldcontained in the MOB_BSHO_REQ message or the MOB_BSHO_RSP messagetransmitted from the serving BS. If 0 has been recorded in theResource_Remain_Type field, step 1006 is performed. In step 1006, theMSS recognizes that the serving BS will delete the connectioninformation of a corresponding MSS. In step 1004, if 1 has been recordedin the Resource_Remain_Type field, step 1008 is performed. In step 1008,the MSS checks whether 0 has been recorded in the Resource_Retain_Timefield contained in the MOB_BSHO_REQ message or the MOB_BSHO_RSP message.As a result of the check, when 0 has been recorded in theResource_Retain_Time field, step 1010 is performed. In step 1010, theMSS recognizes that the serving BS will retain the connectioninformation of the MSS for the time duration pre-negotiated during theperformance of the registration procedure with the serving BS.Meanwhile, in step 1008, when a random value other than 0 has beenrecorded in the Resource_Retain_Time field, step 1012 is performed. Instep 1012, the MSS recognizes that the serving BS will retain theconnection information of the MSS for the time duration recorded in theResource_Retain_Time field. Accordingly, while the serving BS retainsthe connection information of the MSS, when the MSS is handed over tothe serving BS again during communication with another target BS, theMSS can reopen communication with the serving BS by performing only aranging procedure of the network reentry procedure.

FIG. 11 is a flow diagram illustrating a handover operation process ofthe MSS that recognizes whether the serving BS retains the connectioninformation of the MSS in the IEEE 802.16e communication systemaccording to an embodiment of the present invention.

Referring to FIG. 11, in step 1102, the MSS performs the network reentryprocedure with the final target BS. In the course of performing thenetwork reentry procedure, step 1104 is performed. In step 1104, whenthe CINR value of the final target BS is less than the PP_THRESHOLD, theMSS determines if the pingpong effect may occur. As a result of thedetermination, when the MSS recognizes that the pingpong effect does notoccur, step 1102 is performed again. The MSS continuously performs thenetwork reentry procedure. When the MSS recognizes that the pingpongeffect will occur, step 1106 is performed. In step 1106, the MSSdetermines to handover to the serving BS. Then, step 1108 is performed.In step 1108, the MSS determines if the serving BS retains theconnection information of the MSS. Herein, the MSS may determine whetherthe connection information is retained by means of the information(Resource_Remain_Type) obtained through the MOB_BSHO_RSP message or theMOB_BSHO_REQ message received from the serving BS. As a result of thedetermination, when the serving BS retains the connection information ofthe MSS, step 1110 is performed.

In step 1110, the MSS synchronizes with the serving BS. Then, step 1112is performed. In step 1112, the MSS having synchronized with the servingBS inserts a Basic Connection ID (CID) used in communication with theserving BS into a RNG_REQ message for an initial ranging, and transmitsthe RNG_REQ message to the serving BS. Then, step 1114 is performed. Instep 1114, the MSS receives a RNG_RSP message from the serving BS as aresponse for the RNG_REQ message. Then, step 1116 is performed. In step1116, the MSS may omit a procedure relating to a basic capacitynegotiation, an authentication and a registration, and reopencommunication with the serving BS by means of the connection informationretained by the serving BS.

Meanwhile, in step 1108, when the serving BS has deleted the connectioninformation of the MSS, step 1118 is performed. In step 1118, the MSSsynchronizes with the serving BS. Then, step 1120 is performed. In step1120, the MSS transmits the RNG_REQ message to the serving BS andperforms the initial ranging procedure. Then, step 1122 is performed.Herein, the RNG_REQ message transmitted from the MSS includes a CIDhaving been set to 0×0000 as in the case of the general initial rangingprocedure. In step 1122, the MSS receives the RNG_RSP message from theserving BS. Then, step 1124 is performed. In step 1124, the MSS performsthe existing network reentry procedure, that is, the procedure relatingto the basic capacity negotiation, the authentication and theregistration, with the serving BS. Then, step 1126 is performed. In step1126, the MSS having completed the network reentry procedure reopenscommunication with the serving BS.

The RNG_REQ message for the Basic CID contained in the RNG_REQ messagehas an encoding structure as shown in Table 18. TABLE 18 Name TypeLength Values (Variable-length) Basic CID TBD (6) 2 The Basic CIDassigned from the former Serving BS

As shown in Table 18, the encoding structure of the RNG_REQ message forthe Basic CID includes a Type representing that the type of a TLV set inthe RNG_REQ message is a Basic CID, a Length reporting the length (2bytes) of the Basic CID, and a Value representing what the Basic CIDmeans. The Basic CID is Basic CID information used when the MSScommunicates with the serving BS. Herein, it is noted that the Typefield has a value of 6 (random value) and may change afterwards.

According to the present invention as described above, in an IEEE802.16e communication system, which is a BWA communication system usingan OFDM/OFDMA scheme, an MSS can recognize in advance situations inwhich handover may frequently occur between a serving BS and a pluralityof target BSs, that is, situations in which a pingpong effect may occur.That is, a PP_THRESHOLD is set, so that handover to a BS having a CINRvalue less than the PP_THRESHOLD can be prevented. Accordingly, the MSScan maximally prevent the situations in which the pingpong effect mayoccur. Further, the present invention allows BSs to flexibly retain ordelete connection information of the MSS according to channelconditions, so that a network reentry procedure including only aninitial ranging is performed when the MSS is handed over to a finaltarget BS having retained the connection information. Therefore, acommunication service can be quickly reopened.

While the present invention has been shown and described with referenceto certain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A method for performing a handover by a Mobile Subscriber Station(MSS) in a Broadband Wireless Access (BWA) communication systemincluding a serving Base Station (BS) for providing a service to the MSSand a target BS adjacent to the serving BS, the method comprising thesteps of: receiving a handover request message from the serving BS, thehandover request message including a resource remain type fieldrepresenting whether connection information having been set between theserving BS and the MSS is retained; and recognizing whether the servingBS retains the connection information according to a value of theresource remain type field.
 2. The method as claimed in claim 1, furthercomprising the steps of: transmitting a handover indication message tothe serving BS, the handover indication message including informationreporting performance of the handover; and releasing connection with theserving BS upon transmitting the handover indication message recognizingthat retaining of the connection information starts from a point in timeat which the handover indication message is received by the serving BS.3. The method as claimed in claim 1, wherein the handover requestmessage notifies resource retain time information when the resourceremain type field represents retaining of the connection information. 4.The method as claimed in claim 3, wherein the resource retain timeinformation includes a resource retain time field representing a timeduration value in the handover request message.
 5. The method as claimedin claim 3, wherein the resource retain time information includes asystem retaining time representing a pre-negotiated time when aregistration is performed between the serving BS and the MSS.
 6. Themethod as claimed in claim 1, further comprising the steps of:determining an reentry to the serving BS during a handover to the targetBS; and reopening communication with the serving BS using the connectioninformation, when the serving BS retains the connection information. 7.A method for performing a handover by a Mobile Subscriber Station (MSS)in a Broadband Wireless Access (BWA) communication system including aserving Base Station (BS) for providing a service to the MSS and atarget BS adjacent to the serving BS, the method comprising the stepsof: requesting the handover to the serving BS; receiving a handoverresponse message from the serving BS, the handover response messageincluding a resource remain type field representing whether connectioninformation having been set between the serving BS and the MSS isretained; and recognizing whether the serving BS retains the connectioninformation according to a value of the resource remain type field. 8.The method as claimed in claim 7, further comprising the steps of:transmitting a handover indication message to the serving BS, thehandover indication message including information reporting performanceof the handover; and releasing connection with the serving BS upontransmitting the handover indication message recognizing that retainingof the connection information starts from a point in time at which thehandover indication message is received from the serving BS.
 9. Themethod as claimed in claim 7, wherein the handover response messagenotifies resource retain time information when the resource remain typefield represents retaining of the connection information.
 10. The methodas claimed in claim 9, wherein the resource retain time informationincludes a resource retain time field representing a time duration valuein the handover response message.
 11. The method as claimed in claim 9,wherein the resource retain time information includes a system retainingtime representing a pre-negotiated time when a registration is performedbetween the serving BS and the MSS.
 12. The method as claimed in claim7, further comprising the steps of: determining an reentry to theserving BS during a handover to the target BS; and reopeningcommunication with the serving BS using the connection information, whenthe serving BS retains the connection information.
 13. A method forperforming a handover by a serving Base Station (BS) for providing aservice to a Mobile Subscriber Station (MSS) in a Broadband WirelessAccess (BWA) communication system including a target BS adjacent to theserving BS, the method comprising the steps of: transmitting a handoverrequest message to the MSS, the handover request message including aresource remain type field representing whether connection informationhaving been set between the serving BS and the MSS is retained;receiving a handover indication message from the MSS; and retaining theconnection information according to a value of the resource remain typefield.
 14. The method as claimed in claim 13, wherein retaining theconnection information starts from a point in time at which the handoverindication message is received form the MSS, when the value of theresource remain type field represents retaining of the connectioninformation.
 15. The method as claimed in claim 14, wherein the handoverindication message includes a handover indication type representsreleasing the serving BS, that is HO_IND_type=00.
 16. The method asclaimed in claim 13, further comprising a step of determining whetherthe connection information is retained according to the receivedhandover indication message.
 17. The method as claimed in claim 13,wherein the handover request message notifies resource retain timeinformation when the resource remain type field represents retaining ofthe connection information.
 18. The method as claimed in claim 17,wherein the resource retain time information includes a resource retaintime field representing a time duration value in the handover requestmessage.
 19. The method as claimed in claim 17, wherein the resourceretain time information includes a system retaining time representing apre-negotiated time when a registration is performed between the servingBS and the MSS.
 20. The method as claimed in claim 17, furthercomprising a step of reopening communication with the MSS using theconnection information, when the serving BS retains the connectioninformation with the MSS in a reentry of the MSS.
 21. The method asclaimed in claim 17, wherein the connection information with the MSS isdeleted when the value of the resource remain type field does notrepresent retaining of the connection information.
 22. A method forperforming a handover by a serving Base Station (BS) for providing aservice to a Mobile Subscriber Station (MSS) in a Broadband WirelessAccess (BWA) communication system including a target BS adjacent to theserving BS, the method comprising the steps of: receiving a handoverrequest message from the MSS; transmitting a handover response messageto the MSS, the handover response message including a resource remaintype field representing whether connection information having been setwith the MSS is retained; receiving a handover indication message fromthe MSS; and retaining the connection information according to a valueof the resource remain type field.
 23. The method as claimed in claim22, wherein retaining the connection information starts from a point intime at which the handover indication message is received form the MSS,when the value of the resource remain type field represents retaining ofthe connection information.
 24. The method as claimed in claim 23,wherein the handover indication message includes a handover indicationtype represents releasing the serving BS, that is HO_IND_type=00. 25.The method as claimed in claim 23, further comprising a step ofdetermining retaining of the connection information when the receivedhandover indication message includes information reporting performanceof a handover by the MSS.
 26. The method as claimed in claim 23, whereinthe handover response message notifies resource retain time informationwhen the resource remain type field represents retaining of theconnection information.
 27. The method as claimed in claim 26, whereinthe resource retain time information includes a resource retain timefield representing a time duration value in the handover requestmessage.
 28. The method as claimed in claim 26, wherein the resourceretain time information includes a system retaining time representing apre-negotiated time when a registration is performed between the servingBS and the MSS.
 29. The method as claimed in claim 24, furthercomprising a step of reopening communication with the MSS using theconnection information, when the serving BS retains the connectioninformation with the MSS in a reentry of the MSS.
 30. A handover methodfor minimizing a service delay due to a pingpong effect in a BroadbandWireless Access (BWA) communication system including a Mobile SubscriberStation (MSS), a serving Base Station (BS) communicating with the MSS,and a plurality of neighbor BSs, the method comprising the steps of: a)requesting to the serving BS a handover when it is detected thathandover must be performed from the serving BS to a final target BSwhich is one of the neighbor BSs; b) detecting information for at leasttwo candidate target BSs, on of which is selected as the final targetBS, according to the handover request, the information being receivedfrom the serving BS; c) measuring CINR (Carrier to Interference andNoise Ratio) values transmitted from each of the candidate target BSs;d) comparing the measured CINR values with a first predeterminedthreshold value and excluding a first candidate target BS, whichtransmits a reference signal having a CINR value less than the firstthreshold value, from the candidate target BSs when CINR values lessthan the first threshold value exist; and e) determining the finaltarget BS in the candidate target BSs excluding the first candidatetarget BS and notifying the serving BS of a handover to the final targetBS.
 31. The method as claimed in claim 30, wherein step a) comprises astep of detecting that the handover to the final target BS must beperformed when a CINR value of the reference signal transmitted from theserving BS is less than a second threshold value, and the firstthreshold value is set to have a value exceeding the second thresholdvalue.
 32. The method as claimed in claim 30, wherein step e) comprisesa step of determining a candidate target BS, which provides a servicelevel or a bandwidth required for the communication from among thecandidate target BSs excluding the first candidate target BS, as thefinal target BS.
 33. The method as claimed in claim 30, furthercomprising a step of notifying the serving BS that a handover is notperformed when the measured CINRs are less than the first thresholdvalue.
 34. The method as claimed in claim 30, further comprising thesteps of: changing connection from the serving BS to the final target BSafter notifying the serving BS of the handover to the final target BS;performing a network reentry procedure with the final target BS afterchanging the connection from the serving BS to the final target BS; andchanging the connection from the final target BS to the serving BS whena CINR value of a reference signal transmitted from the final target BSis less than the first threshold value while the network reentryprocedure is performed.
 35. A handover method for minimizing a servicedelay due to a pingpong effect in a Broadband Wireless Access (BWA)communication system including a Mobile Subscriber Station (MSS), aserving Base Station (BS) communicating with the MSS, and a plurality ofneighbor BSs, the method comprising the steps of: a) receiving ahandover request including information for at least two candidate targetBSs from the serving BS, one of the candidate target BSs being selectedas a final target BS by the serving BS, the final target BS being one ofthe neighbor BSs, and detecting the information; b) measuring CINR(Carrier to Interference and Noise Ratio) values transmitted from eachof the candidate target BSs; c) comparing the measured CINR values witha first predetermined threshold value and excluding a first candidatetarget BS, which transmits a reference signal having the CINR value lessthan the first threshold value, from the candidate target BSs when CINRvalues less than the first threshold value exist; and d) determining thefinal target BS in the candidate target BSs excluding the firstcandidate target BS and notifying the serving BS of handover to thefinal target BS.
 36. The method as claimed in claim 35, wherein step d)comprises a step of determining a candidate target BS, which providesone of a service level and a bandwidth required for the communicationfrom among the candidate target BSs excluding the first candidate targetBS, as the final target BS.
 37. The method as claimed in claim 35,further comprising a step of notifying the serving BS that a handover isnot performed when the measured CINRs are less than the first thresholdvalue.
 38. The method as claimed in claim 35, further comprising thesteps of: changing a connection from the serving BS to the final targetBS after notifying the serving BS of the handover to the final targetBS; performing a network reentry procedure with the final target BSafter changing the connection from the serving BS to the final targetBS; and changing the connection from the final target BS to the servingBS when a CINR value of a reference signal transmitted from the finaltarget BS is less than the first threshold value while the networkreentry procedure is performed.
 39. A handover method for minimizing aservice delay due to a pingpong effect in a Broadband Wireless Access(BWA) communication system including a Mobile Subscriber Station (MSS),a serving Base Station (BS) communicating with the MSS, and a pluralityof neighbor BSs, the method comprising the steps of: a) one of deletingconnection information of the MSS and retaining the connectioninformation during a preset time, when the serving BS receives anotification representing that the MSS is to be handed over to a targetBS which is a specific neighbor BS of the neighbor BSs; b) changing aconnection to the serving BS when the MSS recognizes an occurrence of apingpong effect during a network reentry procedure with the target BS;c) receiving one of a handover request message and a handover responsemessage from the serving BS; d) detecting and recognizing if theconnection information for the MSS is retained, which is contained ineach message, according to reception of each message; and e) performingonly an initial ranging procedure and reopening communication with theserving BS when the serving BS retains the connection information forthe MSS.
 40. The method as claimed in claim 39, wherein step e)comprises the steps of: synchronizing with the serving BS; transmittinga ranging request message, which corresponds to the initial rangingprocedure including a Basic Connection Identifier (CID) used tocommunicate with the serving BS and, and receiving a ranging responsemessage as a response for the ranging request message; and reopening thecommunication by means of the connection information of the MSS retainedby the serving BS.
 41. The method as claimed in claim 39, furthercomprising a step of reopening the communication with the serving BS byperforming an existing network reentry procedure when the serving BS hasdeleted the connection information of the MSS.
 42. The method as claimedin claim 39, further comprising a step of detecting information forpreset connection information-retaining time when the MSS recognizesthat the serving BS retains the connection information for the MSS. 43.The method as claimed in claim 42, wherein the serving BS retains theconnection information for the MSS according to the information for theconnection information-retaining time.
 44. A handover system forminimizing a service delay due to a pingpong effect in a BroadbandWireless Access (BWA) communication system including an MSS (MobileSubscriber Station), a serving BS (Base Station) for communicating withthe MSS, and a plurality of neighbor BSs, the system comprising: a MSSfor requesting handover to a serving BS when it is detected thathandover must be performed from the serving BS to a final target BSwhich is one of the neighbor BSs, detecting information for at least twocandidate target BSs, which are selected as the final target BS,according to the handover request, the information being received fromthe serving BS, measuring CINR (Carrier to Interference and Noise Ratio)values transmitted from each of the candidate target BSs, comparing themeasured CINR values with a first predetermined threshold value andexcluding a first candidate target BS, which transmits a referencesignal having a CINR value less than the first threshold value, from thecandidate target BSs when CINR values less than the first thresholdvalue exist, and determining the final target BS in the candidate targetBSs excluding the first candidate target BS and notifying the serving BSof handover to the final target BS; and a serving BS for receiving therequest of the MSS, transmitting the information for at least twocandidate target BSs, which are selected as the final target BS by theMSS, and receiving a notification representing that the MSS is to behanded over to the final target BS from the MSS.
 45. The system asclaimed in claim 44, wherein, in detecting that the MSS must be handedover to the final target BS, the handover to the final target BS isdetected when a CINR value of the reference signal transmitted from theserving BS is less than a second threshold value, and the firstthreshold value is set to have a value exceeding a second thresholdvalue.
 46. The system as claimed in claim 44, wherein, in determiningthe final target BS by the MSS, a candidate target BS, which provides aservice level or a bandwidth required for the communication from amongthe candidate target BSs excluding the first candidate target BS, isdetermined as the final target BS.
 47. The system as claimed in claim44, wherein a notification representing that the handover is notperformed is transmitted to the serving BS when the measured CINRs areless than the first threshold value.
 48. A handover system forminimizing a service delay due to a pingpong effect in a BroadbandWireless Access (BWA) communication system including an MSS (MobileSubscriber Station), a serving BS (Base Station) for communicating withthe MSS, and a plurality of neighbor BSs, the system comprising: a MSSfor receiving a handover request including information for at least twocandidate target BSs from a serving BS, the candidate target BSs beingselected as a final target BS by the serving BS, the final target BSbeing one of the neighbor BSs, and detecting the information, measuringCINR (Carrier to Interference and Noise Ratio) values transmitted fromeach of the candidate target BSs, comparing the measured CINR valueswith a first predetermined threshold value and excluding a firstcandidate target BS, which transmits a reference signal having the CINRvalue less than the first threshold value, from the candidate target BSswhen CINR values less than the first threshold value exist, anddetermining the final target BS in the candidate target BSs excludingthe first candidate target BS and notifying the serving BS of handoverto the final target BS; and a serving BS for requesting the handover ofthe MSS and receiving information for the final target BS from the MSS.49. A system for performing a handover in a Broadband Wireless Access(BWA) communication system including a Mobile Subscriber Station (MSS),a serving Base Station (BS) for providing a service to the MSS, and atarget BS adjacent to the serving BS, the system comprising: a MSS forrequesting the handover to the serving BS, receiving a handover responsemessage from the serving BS, the handover response message including aresource remain type field representing whether preset connectioninformation is retained, and recognizing whether the serving BS retainsthe connection information according to a value of the resource remaintype field; and a serving BS for receiving a handover request messagefrom the MSS, transmitting the handover response message to the MSS, thehandover response message including the resource remain type fieldrepresenting whether the connection information having been set with theMSS is retained, receiving a handover indication message from the MSS,and retaining the connection information according to the value of theresource remain type field.
 50. A system for performing a handover in aBroadband Wireless Access (BWA) communication system including a MobileSubscriber Station (MSS), a serving Base Station (BS) for providing aservice to the MSS, and a target BS adjacent to the serving BS, thesystem comprising: a MSS for receiving a handover request message fromthe serving BS, the handover request message including a resource remaintype field representing whether connection information having been setwith the serving BS is retained, and recognizing whether the serving BSretains the connection information according to a value of the resourceremain type field; and a serving BS for transmitting the handoverrequest message to the MSS, the handover request message including theresource remain type field representing whether the connectioninformation having been set with the MSS is retained, receiving ahandover indication message from the MSS, and retaining the connectioninformation according to the value of the resource remain type field.